This invention relates to a method of depositing thin film particularly metallic or dielectric films in the fabrication of integrated circuits.
At present in the semiconductor industry the formation of thin film patterns atop semiconductor substrates is accomplished primarily by etching in the presence of etch-resistant photoresist layers. The process involves the traditional photolithographic wetetching of both the thin film as well as the photoresist layers.
These relatively old and well-known techniques have been eminently successful. However, with the continued miniturization of semiconductor integrated circuits to achieve greater component density and smaller units of large scale integrated circuitry, the art is rapidly approaching a point where either optical processing or wet etching or both may be impractical for providing the minute resolution required for the fine line definition of metallization.
Wet etching of thin films may be used in either electron beam or optical exposure systems. However, it often results in the contamination of the metal, primarily due to particles within the etching solution itself. In addition to the purity and composition of the etchant, the duration of the etching must be carefully controlled to prevent under or over-etching of the thin film. Plasma, or reactive ion, etching has in relatively recent times come to be recognized as a practical alternative to wet etching. Contamination may be less of a problem and the etching equipment assures adequate process control for the most precise thin film patterns.
One of the problems associated with plasma etching of thin films, however, is that most of the well-known and commonly used optical-and electron beam-resist materials cannot withstand the processing intact. The resists tend to flow during the etching process apparently because of their reactions with the gaseous ions and the temperature of the semiconductor substrate, typically around 200.degree. C or more. Thus, it would be desirable to be able to plasma etch a single photoresist material such as polymethylmethacrylate (PMMA) directly over a metallic thin film to define the desired conductive pattern. The resist would then be used as a mask and the exposed metal etched away, typically by another gas which attacks the metal but not the photoresist. The remaining photoresist would then be removed in the conventional manner to leave the desired thin film pattern.
To our knowledge there is no practical way to accomplish this single-coat technique of plasma etching thin films. A number of different resist materials and reactive gases have been tried, but with little success.
This problem exists with materials which are both optically and electron sensitive such as AZ-1350J, a positive resist available from the Shipley Corp. The same holds for negative resist materials which are electron or optically sensitive, such as KTFR or KMER which are available from the Eastman Kodak Corporation.
The problem is particularly acute with highly sensitive resist materials such as PMMA, which is an excellent electronsensitive material. It has been recognized for many years that electron beam sytems would be more advantageous than optical systems for the exposure of photoresist layers. Electron beam systems offer greater resolution and power density then optical systems; and electron beams can be deflected electrically. Until recently, the use of electron beam systems has been inhibited by the lack of high quality resist material and the development and tooling cost of adequate electron beam systems. However, it appears that electron beam techniques are on the verge of substantially replacing optical systems. It is therefore highly desirable to develop an E-beam resist system which is compatable with plasma etching.